A single mode optical fiber comprising a core made of pure quartz glass and a cladding made of fluorine added quartz glass theoretically has small attenuation of light transmission since the core does not contain any additive which causes scattering loss. Since pure quartz glass has less stuctural defects, it has good resistance to irradiation, which varies with the amount of the defects. Therefore, once such optical fiber is fabricated, it can be used as a reliable optical fiber suitable for telecommunication with a large capacity. However, such optical fiber is difficult to produce. For example, when it is produced by a known modified chemical vapor deposition (MCVD) method, it is difficult to increase a synthesizing rate of the cladding made of the fluorine added quartz glass, which means poor productivity of the optical fiber. By a vapor phase axial deposition (VAD) method, it is difficult to form a refractive index distribution by the addition of fluorine.
As one of methods for producing a glass preform for use in the fabrication of an optical fiber, a so-called rod-in-collapse method is known. The rod-in-collapse method comprises inserting, as a core material, a glass rod having a higher refractive index in a quartz glass tube having a lower refractive index than that of the core material and heating and fusing them together, namely collapsing them to form a glass preform. However, by the rod-in-collapse method, it is difficult to reduce the attenuation of light transmission while it is easy to form a refractive index distribution.
As the results of the study of the present inventors, followings have been found:
For the production of a glass preform from which a single mode optical fiber having a larger ratio of a cladding diameter to a core diameter, it is difficult to form a cladding layer with a desired thickness in a single step by the rod-in-collapse method. That is, when the core rod has a small outer diameter, it is easily deformed in the collapsing step so that the core portion of the resulting glass preform tends to be eccentric. When the core rod has a large outer diameter, the wall thickness of the quartz tube should be made large. Therefore, it is rather difficult to collapse them.
It is proposed to collapse a plural of tubes made of quartz glass added with fluorine successively around the core rod made of pure quartz glass. By this method, eccentricity of the core is suppressed or prevented and all the cladding layers can be formed with good thermal efficiency. When an optical fiber is fabricated from the glass preform produced by this method, absorption loss of the optical fiber due to, for example, residual hydroxyl groups can be decreased by suitable treatment of the preform before or during collapsing. However, it is found that the optical fiber still has some attenuation of light transmission which is independent on a wavelength. Further, in the above method, since the several collapsing steps are required, a large amount of materials is wasted so that the productivity of the final optical fiber is not sufficient.